Recent years have seen upgrading in the quality of digital data such as videos, photographs, and the like, and conversion of paper media into electronic forms, and so on, which have resulted in drastic increases in their data amounts. In particular, in models called cloud computing, where various applications and various services are offered for use via servers, storages, and the like which are on a network, large numbers of users save various data in the storages on the network. This results in huge amounts of data accumulation.
Conventionally, magnetic tape apparatuses have often been used as storages for saving. In their place, optical tape apparatuses have been proposed which simultaneously perform recording and reproduction on an optical tape medium with a plurality of optical pickups, by utilizing a high-density recording technique based on light. For example, Patent Document 1 discloses an example of such an optical tape apparatus.
In the conventional magnetic tape apparatuses, a write head and a read head are separately provided for a track in which data is to be recorded. While recording data with the write head, the data which has been recorded is reproduced with the read head, thus making it possible to check (verify) whether the data has been correctly recorded or not. By doing so, magnetic tape apparatuses ensure reliability of data recording.
Also in optical recording/reproduction apparatuses which perform recording/reproduction for optical discs by using light, techniques of performing a verify are known. Such techniques are called DRAW (Direct Read After Write). An optical recording/reproduction apparatus which utilizes the DRAW technique employs a diffraction grating to split a light beam having been emitted from a laser light source into a 0th order light beam and ±1st order light beams, and allow these split light beams to be radiated onto a recording layer of an optical disc. Data recording is performed with the irradiation of 0th order light, and a verify is performed by detecting the ±1st order light which has been reflected from the optical disc.
With the DRAW technique, immediately after a recorded mark is formed on a track with the 0th order light beam, that recorded mark is error-checked. This attains an enhanced processing speed relative to performing an error check only after recording has entirely been completed. As a result, the data transfer rate of the optical recording/reproduction apparatus can be increased. Recording/reproduction apparatuses utilizing the DRAW technique are disclosed in Patent Document 2, for example.
On the other hand, in an optical recording/reproduction apparatus, it is necessary to form a write beam spot and a read beam spot on a recording track which is within a recording layer. In order to maintain these beam spots at appropriate positions with a high accuracy, tracking control and focus control must be performed. Magnitudes of defocus and mistracking are indicated by, respectively, a “focus error signal” and a “tracking error signal” which are generated based on reflected light from the optical storage medium.
As a focus control method in conventional optical disc apparatuses, the astigmatic method has long been known, for example, which is used in many optical disc apparatuses even nowadays. Other known methods are the spot size detection method (SSD method) and the like. Focus control based on the SSD method is disclosed in Patent Document 3, for example.
On the other hand, as tracking control methods in conventional optical disc apparatuses, the push-pull method (PP method), the advanced push-pull method (APP method), and the correct far field method (CFF method) are known, for example. Tracking control based on the PP method, the APP method, and the CFF method, respectively, is disclosed in Patent Documents 4 to 6.